Keratoconus is typically characterized by marked irregular corneal astigmatism and corneal thinning, resulting in long-term dependence on rigid gas permeable contact lenses for correcting the corneal irregularity and producing usable vision. Keratoconus is diagnosed typically during young adulthood, and the disease compromises vision during the patients' prime education and earning years. It is one of the most common diseases managed by cornea specialists and accounts for much of the time spent by contact lens practitioners. Currently employed management methods typically include, successively, spectacle correction, rigid contact lenses, and penetrating keratoplasty as the disease progresses. The objective of the Collaborative Longitudinal Evaluation of Keratoconus (CLEK) Study is to characterize the progression of keratoconus over a broad spectrum of disease severity. The Study will include patients 12 through 65 years of age who have unilateral corneal irregularity and evidence of either Vogt's striae, Fleischer's ring, or corneal scarring in at least one eye. The progression of keratoconus will be measured by changes in visual acuity (high and low contrast Bailey-Lovie with best correction, habitual correction, and manifest refraction), visual quality of life, corneal curvature (keratometry), the contact lens base curve required for definite apical clearance of the cornea, and central corneal scarring. Thirteen Participating Clinics will recruit and follow a total of 1,000 keratoconus patients. CLEK Study patients will be examined annually for three years. Resource centers for the CLEK Study are the Study Chairman's Office at the University of California at Berkeley School of Optometry, Berkeley, California, the CLEK Data Coordinating Center at Washington University Medical School, St. Louis, Missouri, and the CLEK Photography Reading Center at The Ohio State University College of Optometry, Columbus, Ohio.